1. Field of the Invention
This invention relates generally to the field of insulator coatings, and specifically to a superhydrophobic surface coating for use as a protective coating for power systems.
2. Description of Related Art
Conventional high-voltage devices such as bushings, connectors, and capacitors use a combination of non-conductive and conductive materials to construct desired high-voltage structures. The nonconductive materials provide a dielectric barrier or insulator between two electrodes of different electrical potential.
The bulk of power delivery from the generating sites to the load centers is accomplished by overhead lines. To minimize line losses, power transmission over such long distances is more often carried out at high voltages (several hundred kV). The energized high voltage (HV) line conductors not only have to be physically attached to the support structures, but also the energized conductors have to be electrically isolated from the support structures. The device used to perform the dual functions of support and electrical isolation is the insulator.
High voltage insulators are used with transmission and distribution systems, including power transmission lines, for example at locations where the lines are suspended. Known insulators include ceramics, glass and polymeric materials. Ceramic and glass insulators have been used for over 100 years. The widespread use of polymeric insulators began in North America during the 1970s. A currently popular line of insulators are room temperature vulcanized (RTV) silicone rubber high voltage insulator coatings.
Ceramic insulators generally include clay ceramics, glasses, porcelains, and steatites. The ceramic is produced from the starting materials kaolin, quartz, clay, alumina and/or feldspar by mixing the same while adding various substances in a subsequent firing or sintering operation. Polymeric materials include composites (EPDM rubber and Silicone rubber) and resins.
A wide variety of manufacturing techniques can be employed to construct insulators of the desired shape. Some of the processes that are most often used include machining, molding, extrusion, casting, rolling, pressing, melting, painting, vapor deposition, plating, and other free-forming techniques, such as dipping a conductor in a liquid dielectric or filling with dielectric fluid. The selection process must take into account how one or both of the electrodes made from conductive material will be attached or adjoined to the insulator.
In long-term use, an insulator is subject to a greater or lesser degree of superficial soiling, depending on the location at which it is used, which can considerably impair the original insulating characteristics of the clean insulator. Such soiling is caused for example by the depositing of industrial dust or salts or the separating out of dissolved particles during the evaporation of moisture precipitated on the surface. In many parts of the world, insulator contamination has become a major impediment to the supply of electrical power. Contamination on the surface of insulators gives rise to leakage current, and if high enough, flashover.
One problem afflicting high voltage insulators used with transmission and distribution systems includes the environmental degradation of the insulators. Insulators are exposed to environment pollutants from various sources. It can be recognized that pollutants that become conducting when moistened are of particular concern. Two major sources of environmental pollution include coastal pollution and industrial pollution.
Coastal pollution, including salt spray from the sea or wind-driven salt-laden solid material such as sand, can collect on the insulator's surface. These layers become conducting during periods of high humidity and fog. Sodium chloride is the main constituent of this type of pollution.
Industrial pollution occurs when substations and power lines are located near industrial complexes. The power lines are then subject to the stack emissions from the nearby plants. These materials are usually dry when deposited, then may become conducting when wetted. The materials will absorb moisture to different degrees. Apart from salts, acids are also deposited on the insulator.
Of course, both sources of pollution can exist. For example, if a substation is situated near to the coast, it will be exposed to a high saline atmosphere together with any industrial and chemical pollution from other plants situated in close proximity.
The presence of a conducting layer on the surface of an insulator can lead to pollution flashover. In particular, sufficient wetting of the dry salts on the insulator surface is required to from a conducting electrolyte. The ability of a surface to become wet is described by its hydrophobicity. Ceramic materials and some polymeric materials such as EDPM rubber are hydrophilic, that is, water films out easily on its surface. In the case of some shed materials such as silicone rubber, water forms beads on the surface due to the low surface energy.
When new, the hydrophobic properties of silicone rubber are excellent; however, it is known that severe environmental and electrical stressing may destroy this hydrophobicity.
Current remediation techniques for environmental degradation of a high voltage insulator include washing, greasing and coatings, among others. Substation or line insulators can be washed when de-energized or when energized. Cleaning with water, dry abrasive cleaner, or dry ice can effectively remove loose contamination from insulator, but it is expensive and labor intensive. It is not uncommon that washings involve shutting down the power once every two weeks in winter time and once per week in summer when doing this kind of maintenance. This common occurrence of de-energization simply is not preferable.
Mobile protective coatings, including oils, grease and pastes surface treatment, can prevent flashover, but have damaging results to the insulator during dry band arcing. A thin layer of silicone grease, when applied to ceramic insulators, increases the hydrophobicity of the surface. Pollution particles that are deposited on the insulator surface are also encapsulated by the grease and protected from moisture. A disadvantage of greasing is that the spent grease must be removed and new grease applied, typically annually. Grease-like silicone coating components, usually compounded with alumina tri-hydrate (ATH), provide a non-wettable surface and maintain high surface resistance. Thus, greasing can greatly reduce maintenance costs when viewed against washings, but the substation has to remove the old grease compounds from the equipment, and then re-apply the new grease compound annually.
Fluorourethane coatings were developed for high voltage insulators, but the field test is not successful, and its adhesion to insulators has been a problem.
Since the 1970s, silicone room temperature vulcanizing (RTV) coatings have gained considerable popularity, and become the major products available in the market, such as Dow Corning's SYLGARD High Voltage Insulator Coatings, CSL's Si-Coat HVIC, and Midsun's 570 HVIC. Service experience has indicated that of the various types of insulator coatings, the time between maintenance and RTV coating reapplication is the longest.
Room temperature cured silicone rubber coatings are available to be used on ceramic or glass substation insulators. These coatings have good hydrophobic properties when new. Silicone coatings provide a virtually maintenance-free system to prevent excessive leakage current, tracking, and flashover. Silicone is not affected by ultraviolet light, temperature, or corrosion, and can provide a smooth finish with good tracking resistance.
Silicon coatings are used to eliminate or reduce regular insulator cleaning, periodic re-application of greases, and replacement of components damaged by flashover. They appear to be effective in many types of conditions, from salt-fog to fly ash. They are also useful to restore burned, cracked, or chipped insulators.
SYLGARD is one type of silicone coatings, and is marketed to restrict the rise in leakage currents and protect the insulators against pollution induced flashovers. The cured SYLGARD coating has a high hydrophobicity. This hydrophobic capability is of prime importance because it is this factor that controls the degree of wetting of the contaminants, and thereby the amount of surface leakage current increase. Moisture on the insulator surface will form in droplets and by so doing will prevent the surface pollution from becoming wet and producing a conductive layer of ionisable materials that lead to increased leakage, dry band arcing and eventual flashovers.
In addition, there are a certain percentage of polymer molecules that exist within the cured rubber as low molecular weight free fluid. These molecules are known as “cyclics”. The free fluids are easily able to migrate to the surface of the coating and, as pollutants fall on the surface, they in turn are encapsulated and rendered non conductive and somewhat hydrophobic.
If leakage currents are controlled, there will be no arcing. If there is an extreme weather event then it may be that, for a time, the SYLGARD coating cannot control the surface leakage currents. In this case SYLGARD also provides a high degree of surface arc resistance. Incorporated into the formulation is an alumina trihydrate (ATH) filler, which releases H2O when it becomes hot and consequently resists the degradative effects of high temperatures, resulting from exposure of the coating to arcing.
However, none of the above techniques prevent contamination, such as dust, accumulation on coating surfaces, and none of the above techniques has satisfactory performance in heavy contamination environments.
Although high voltage insulator coatings are known, as discussed above, a need yet exists for a superior product that can minimize the maintenance necessary for conventional coatings. An HVIC that is self-cleaning and has an expected longer life than conventional coatings would be beneficial.
The abovementioned criteria are satisfied in the natural world. The phenomenon of the water repellency of plant leaf surfaces has been known for many years. The Lotus Effect is named after the lotus plant. The Lotus Effect implies two indispensable characteristic properties: superhydrophobicity and self-cleaning. Superhydrophobicity is manifested by a water contact angle larger than 150°, while self-cleaning indicates that particles of dirt such as dust or soot are picked up by the drop of water as they roll off and removed from the surface.
It is recognized that when a water drop is placed on a lotus plant surface, the air entrapped in the nano surface structures prevents the total wetting of the surface, and only a small part of the surface, such as the tip of the nano structures, can contact with the water drop. This enlarges the water/air interface while the solid/water interface is minimized. Therefore, the water gains very little energy through adsorption to compensate for any enlargement of its surface. In this situation, spreading does not occur, the water forms a spherical droplet, and the contact angle of the droplet depends almost entirely on the surface tension of the water.
Although the Lotus Effect was discovered in plants, it is essentially a physicochemical property rather than a biological property. Therefore, it is possible to mimic the lotus surface structure. To mimic the lotus surfaces, a Lotus Effect surface should be produced by creating a nanoscale rough structure on a hydrophobic surface, coating thin hydrophobic films on nanoscale rough surfaces, or creating a rough structure and decreasing material surface energy simultaneously. Up to now, many methods have been developed to produce hydrophobic surfaces with nano-scale roughness.
Thus, surfaces with a combination of microstructure and low surface energy are known to exhibit interesting properties. A suitable combination of structure and hydrophobicity renders it possible that even slight amounts of moving water can entrain dirt particles adhering to the surface and clean the surface completely. It is known that if effective self-cleaning is to be obtained on an industrial surface, the surface must not only be very hydrophobic but also have a certain roughness. Suitable combinations of structure and hydrophobic properties permit even small amounts of water moving over the surface to entrain adherent dirt particles and thus clean the surface. Such surfaces are disclosed in, for example, WO 96/04123 and U.S. Pat. No. 3,354,022).
European Pat. No. 0 933 380 discloses that an aspect ratio of >1 and a surface energy of less than 20 mN/m are required for such self-cleaning surfaces. The aspect ratio is defined to be a quotient of a height of a structure to a width of the structure.
Other prior art references include PCT/EP00/02424, that discloses that it is technically possible to render surfaces of objects artificially self-cleaning. The surface structures, composed of protuberances and depressions, required for the self-cleaning purpose have a spacing between the protuberances of the surface structures in the range of 0.1 to 200 μm and a height of the protuberances in the range from 0.1 to 100 μm. The materials used for this purpose must consist of hydrophobic polymers or a durably hydrophobized material. Detergents must be prevented from dissolving from the supporting matrix. As in the documents previously described, no information is given either on the geometrical shape or radii of curvature of the structures used.
EP 0 909 747 teaches a process for producing a self-cleaning surface. The surface has hydrophobic elevations of height from 5 to 200 μm. A surface of this type is produced by applying a dispersion of powder particles and of an inert material in a siloxane solution, followed by curing. The structure-forming particles are therefore secured to the substrate by an auxiliary medium.
Methods for producing these structured surfaces are likewise known. In addition to molding these structures in a fashion true to detail by way of a master structure using injection molding or by an embossing method, methods are also known which use the application of particles to a surface (e.g. see U.S. Pat. No. 5,599,489). This process utilizes an adhesion-promoting layer between particles and bulk material. Processes suitable for developing the structures are etching and coating processes for adhesive application of the structure-forming powders, and also shaping processes using appropriately structured negative molds.
However, it is common to all these methods that the self-cleaning behavior of these surfaces is described by a very high aspect ratio.
Plasma technologies are widely utilized for processing of polymers, such as deposition, surface treatment and etching of thin polymer films. The advantages of using plasma techniques to prepare the Lotus Effect coating include that plasma technologies have been extensively employed in surface treatment processes in the electronic industry. Fabricating the Lotus Effect coating on various surfaces with plasma can be easily transferred from research to scale up production. Further, plasma-based methods can be developed into a standard continuous/batch process with low cost, highly uniform surface properties, high reproducibility and high productivity.
Exposure to sunlight and some artificial lights can have adverse effects on the useful life of polymer coatings. UV radiation can break down the chemical bonds in a polymer. Since photodegradation generally involves sunlight, thermal oxidation takes place in parallel with photooxidation. The use of antioxidants during processing is not sufficient to eliminate the formation of photoactive chromospheres. UV stabilizers have been applied widely and the mechanism of stabilization of UV stabilizers belong to one or more of the following: (a) absorption/screening of UV radiation, (b) deactivation (quenching) of chromophoric excited states, and (c) free-radical scavengers, and (d) peroxide decomposers.
Since transmission lines are often in remote locations that are hard to reach, it is desirable that once the line has been constructed, it will work satisfactorily, without maintenance, for the expected life of the line, generally exceeding 30 years. Therefore, it can be seen that a need yet exists for a superior HVIC that utilizes a coating surface exhibiting “Lotus Effect” properties, including superhydrophobicity and self-cleaning.